The present invention relates to a buffer circuit, and more particularly to a low cost buffer circuit having the capability of discharging static electricity.
In an integrated circuit (IC), an input/output (I/O) buffer is utilized for driving an output signal of the integrated circuit to other electronic devices, such as another integrated circuit, via an I/O pad of the I/O buffer. Normally, the driving capability of the I/O buffer is dependent on the size of the I/O buffer. For example, the width of the transistor forming the I/O buffer may need to be as large as a few hundred micrometers to provide enough driving capability for the output signal. In addition, since the I/O pad of the I/O buffer is exposed to the external environment when the I/O buffer is fabricated, the capability to discharge the static electricity is another concern of the I/O buffer. To meet the requirement of the ESD (Electrostatic Discharge) specification, the foundry that manufactures the integrated circuit further standardizes the ESD rules of the transistor forming the I/O buffer in order to sustain the high current produced by the static electricity. Normally, the ESD rules of the transistor can be significantly larger than the normal DRC (Design Rule Check) rules of the core transistor in the integrated circuit. Accordingly, the conventional I/O buffer occupies a large part of area in the integrated circuit and consequently increases the cost of the integrated circuit. Therefore, providing a small size I/O buffer for the integrated circuit while maintaining the capability to drive the output signal and discharge static electricity is a significant concern in the semiconductor industry.